Intermediate filaments (IFs) are a diverse family of cytoskeletal proteins, classified into six Types on the basis of sequence homologies. Types I and II are keratins, with the "hard" `-keratins coming from epidermal specializations such as hair and horn and the "soft" `-keratins expressed in other tissues. Previous STEM measurements of mass/length on IFs showed a quantized polymorphism but were done mostly on IFs assembled in vitro from subunits isolated under denaturing conditions so any physiological implications were unclear. A source of native "hard" `-keratin IF isolated from rat whisker follicles has motivated a reinitiation of these studies. In additon, x-ray diffraction studies and sequence-based model-building suggest that "hard" `-keratin filaments differ from other IFs.